http://graffletopia.com/search/iphone
There are quite a few of them. And to learn to use OmniGraffle, which
appears to be much better than Visio for Windows, there are some great
training videos here:
http://www.omnigroup.com/products/omnigraffle/videos/
You may want to try it out. There is a 14 day trial for OmniGraffle
and the standard version is $100 and the pro version is $200. The pro
version can read in Visio files.
http://informationarchitects.jp/ipad-stencil-for-omnigraffle/
I use OmniGraffle for my Mac, iPhone, and now iPad interface design.
______________________
Brad Larson, Ph.D.
Sunset Lake Software
http://www.sunsetlakesoftware.com